The present invention relates to a method of mass spectrometry and a mass spectrometer. It has become common practice to analyse proteins by first enzymatically or chemically digesting the protein and then analysing the peptide products by mass spectrometry. The mass spectrometry analysis of the peptide products normally entails measuring the mass of the peptide products. This method is sometimes referred to as “peptide mapping” or “peptide fingerprinting”.
It is also known to induce parent or precursor peptide ions to fragment and to then measure the mass of one or more fragment or daughter ions as a way of seeking to identify the parent or precursor peptide ion. The fragmentation pattern of a peptide ion has also been shown to be a successful way of distinguishing isobaric peptide ions. Thus the mass to charge ratio of one or more fragment or daughter ions may be used to identify the parent or precursor peptide ion and hence the protein from which the peptide was derived. In some instances the partial sequence of the peptide can also be determined from the fragment or daughter ion spectrum. This information may be used to detennine candidate proteins by searching protein and genomic databases.
Alternatively, a candidate protein may be eliminated or confirmed by comparing the masses of one or more observed fragment or daughter ions with the masses of fragment or daughter ions which might be expected to be observed based upon the peptide sequence of the candidate protein in question. The confidence in the identification increases as more peptide parent or precursor ions are induced to fragment and their fragment masses are shown to match those expected.